1. Field of the Invention
The present invention relates to a method for operating a compression ignition internal combustion engine.
2. Description of the Related Art
In recent years, in order to reduce a predetermined load, fuel consumption per predetermined time and a quantity of emission matter, in the internal combustion engine, a compression ignition internal combustion engine represented by a homogeneous-charged compression ignition internal combustion engine has been studied. The above described compression ignition internal combustion engine introduces an oxygen-containing gas and a compression self-ignitable fuel into a cylinder, compresses them, and then self-ignites the fuel.
However, it is difficult for the above described homogeneous-charged compression ignition internal combustion engine to produce homogeneous-charged compression ignition combustion in all of the load regions, and it is studied to use diesel combustion when a demand load of the engine is high. Here, the above described homogeneous-charged compression ignition internal combustion engine is a system of “finishing the injection of fuel before the piston reaches a compression top dead center and while a crank angle is larger than 60 degrees”, and the above described diesel internal combustion engine is a system of “spraying fuel while the piston is located approximately between 30 degrees by a crank angle before reaching the compression top dead center and about 10 degrees by the crank angle after having passed the compression top dead center” (see Japanese Patent Laid-Open No. 2004-91657).
In addition, the above described homogeneous-charged compression ignition combustion is a combustion system peculiar to the above described homogeneous-charged compression ignition internal combustion engine, and the above described diesel combustion is the combustion system peculiar to the above described diesel internal combustion engine. Generally, it is preferable in the above described homogeneous-charged compression ignition combustion to use a fuel with a low ignitability (long ignition-delayed period), and in the above described diesel combustion to use a fuel with high ignitability (short ignition-delayed period).
In the next place, reference fuels with an octane number of 50 to 90 were prepared by mixing iso-octane and n-heptane, and on each reference fuel, ignition-delayed periods of time in the diesel combustion and in homogeneous-charged compression ignition combustion were measured. The results are shown in Table 1.
Here, the ignition-delayed period in the above described diesel combustion is a value obtained by measuring a period necessary for a pressure in a vessel to increase by 0.02 MPa, after the fuel has been injected into air heated to 500° C. in the vessel that had been pressurized to 2.0 MPa. On the other hand, the ignition-delayed period in the above described homogeneous-charged compression ignition combustion is a value obtained by measuring a period necessary for a pressure in a combination chamber to increase from an initial pressure (0.1 MPa) to 3.0 MPa, when the fuel premixed at an equivalence ratio (φ) of 0.2 was injected into the combustion chamber having the bore of 80 mm, the stroke of 80 mm and the volume of 402.2 ml, and was compressed at the compression ratio of 13.9 in a condition for the piston to require 53 milliseconds after the pressure has increased by 0.03 MPa from the initial pressure (0.1 MPa), and before the compression is finished.
TABLE 1Ignition-delayed timeFuel composition(ms)(vol. %)DieselHCCIiso-octanen-heptanecombustioncombustionOctane901012.21123.2number 90Octane802010.5975.0number 80Octane70309.3566.4number 70Octane60408.3360.2number 60Octane50507.5757.2number 50Octane40605.6753.2number 40Octane 0100 5.6750.6number 0HCCI combustion: homogeneous-charged compression ignition combustion
It is understood from Table 1 that the reference fuel with an octane number of 80 to 90 has a too long ignition-delayed period for the diesel combustion, but has a suitable ignition-delayed period for the homogeneous-charged compression ignition combustion. In contrast, it is understood that the reference fuel with an octane number of 50 to 70 has a suitable ignition-delayed period for the diesel combustion, but has a too short ignition-delayed period for the homogeneous-charged compression ignition combustion.
Accordingly, in order to produce the above described homogeneous-charged compression ignition combustion when the demand load of the engine is low, in the above described homogeneous-charged compression ignition internal combustion engine, and to produce the above described diesel combustion when the demand load of the engine is high, it is obviously necessary to provide two fuels with an octane number of 80 to 90 for the above described homogeneous-charged compression ignition combustion and an octane number of 50 to 70 for the above described diesel combustion, respectively, and to select one of them for appropriate use.
Conventionally, it has been known that two fuels are provided and selected depending on the demand load of the engine, in the above described homogeneous-charged compression ignition internal combustion engine (see Japanese Patent Laid-Open No. 2004-76736).
However, in order to provide the two fuels and appropriately use them, there is inconvenience of needing to provide a plurality of tanks for separately accommodating each fuel therein.